1. Field of the Invention
The present invention is generally related to the field of semiconductor processing, and, more particularly, to a method and apparatus for controlling the amount of light energy delivered to a layer of photoresist on a semiconductor device.
2. Description of the Related Art
In general, semiconductor devices are manufactured by forming many process layers comprised of various materials above a semiconducting substrate, and, thereafter, removing selected portions of the layers, i.e., patterning the layers. This patterning may be accomplished using known photolithography and etching processes to define the various features of the device, e.g., a gate insulation layer, a gate electrode, sidewall spacers, metal lines and contacts, etc. This forming and patterning of the process layers is typically performed layer by layer as the individual layers are formed, although multiple layers may be patterned at any given time.
Photolithography is a common process used in patterning these various layers. Photolithography typically involves the use of a product known as photoresist. In general terms, photoresist is a product that may be changed from a relatively soluble state to a relatively insoluble state by exposure to a light source. There are positive and negative photoresist currently available on the market.
The photolithography process generally involves forming a layer of photoresist above a previously formed process layer, and exposing selected portions of the layer of photoresist to a light source to form a pattern in the photoresist. The pattern formed in the photoresist is subsequently transferred to the underlying process layer. All of these steps are typically performed in well-known photolithography modules that include a section for depositing the photoresist on the wafer, e.g., a spin-coating station, a device for selectively exposing portions of the photoresist layer to a light source through a reticle, e.g., a stepper, and a section for rinsing and developing the photoresist layer after it has been selectively exposed to the light source. Thereafter, an etching process, such as a plasma etching process, is performed to remove portions of the underlying process layer that are not covered by the patterned layer of photoresist, i.e., the patterned layer of photoresist acts as a mask. After the etching process is complete, the patterned photoresist layer is typically removed so that additional process layers may be formed above the now patterned process layer.
The purpose of the photoresist application step is to form a thin, uniform, defect-free film of photoresist above the substrate surface. A typical layer of photoresist may have a thickness varying from approximately 1500-15,000 xc3x85, and it usually is required to have a uniformity of xc2x1100 xc3x85. Typically, the photoresist is developed by exposing it to a light source of a preselected intensity for a preselected duration of time. Overexposure or underexposure may have undesirable effects on the developed layer of photoresist. That is, dimensions of the patterns formed in the photoresist may be affected by other than ideal exposure. This dimensional variation may carry over to the features that are to be formed in the semiconductor device, and, thus, affect the operation of the semiconductor device, or in the worst case render it inoperable.
Steppers commonly include a light source that is normally on, and a shutter positioned between the light source and the semiconductor device. Thus, exposure of the semiconductor device to the light source is controlled by opening and closing the shutter. For a given light intensity, the duration that the shutter needs to be open may be readily calculated or otherwise derived. However, the light source tends to degrade over time, usually resulting in the shutter being held open for longer and longer periods of time. Typically, a photodose sensor, such as a photodiode, is disposed in the stepper near the semiconductor device. The photodose sensor measures the intensity of the light source, which can be used to determine a corresponding duration for which the shutter is held open.
Unfortunately, the photodose sensor, like the light source, also tends to degrade over time. That is, repeated exposure of the photodose sensor to the light source reduces the magnitude of its output in response to exposure to the same light intensity. Degradation of the photodose sensor may result in the shutter being held open for longer periods of time, overexposing the layer of photoresist, which may adversely affect feature size, particularly critical dimensions (CD).
The present invention is directed to a method of solving or at least reducing some or all of the aforementioned problems.
In one embodiment, the present invention is directed to a method. The method comprises energizing a light source to provide light having a preselected intensity. A first photosensor, which is capable of delivering a first signal indicative of the intensity of the light source, is exposed to the light source. A second photosensor, which is also capable of delivering a second signal indicative of the intensity of the light source, is also exposed to the light source. Thereafter, the first and second signals are compared, and an error signal is delivered in response to detecting a significant difference between the first and second signals.
In another embodiment of the instant invention a method for controlling a stepper is provided. The method comprises providing a wafer having a layer of photoresist disposed thereon, and energizing a light source to deliver light having a preselected intensity onto the layer of photoresist for a preselected duration of time. A first photosensor is exposed to the light source, wherein the first photosensor is capable of delivering a first signal indicative of the intensity of the light source. Similarly, a second photosensor is exposed to the light source, wherein the second photosensor is capable of detecting the intensity of the light source. Thereafter, the first and second signals are compared to determine a difference therebetween, and the preselected duration of time is determined as a function of the difference between the first and second signals.
In still another embodiment of the instant invention, a stepper is provided. The stepper comprises a light source capable of providing light having a preselected intensity. A first photosensor is capable of delivering a first signal indicative of the intensity of the light source. A second photosensor is also capable of delivering a second signal indicative of the intensity of the light source. A controller is adapted to compare the first and second signals, and deliver an error signal in response to detecting a significant difference between the first and second signals.